Facebook: Where can I edit the description text?

You need to change this in the meta description of the page to share. It's here

<meta property="og:description" content="description" />

General: I dont want to share a file

No Problem. You can share whatever you want. Just link to whatever you want. E.g. http://wwww.domain.com/thefile.php
thefile.php can redirect to whatever you want.

General: I want to customize the download

No Problem. Use php sessions. Set the session parameter when the user reaches the page with the pwat-button. (You can set a session via ajax aswell) Link to a page, not a file - and check for the session parameter.

Can't find a solution here :/

No Problem. Just drop us a mail